linux-kernel - Re: [Xen-devel] [PATCH 3/3] xen/blk[front|back]: Enhance discard support with secure erasing support.

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Date:	Mon, 10 Oct 2011 20:19:15 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	Jan Beulich <JBeulich@...ell.com>,
	Li Dongyang <lidongyang@...ell.com>
Subject: Re: [Xen-devel] [PATCH 3/3] xen/blk[front|back]: Enhance discard
 support with secure erasing support.

Where is your tree at the moment?

On Mon, 2011-10-10 at 18:53 +0100, Konrad Rzeszutek Wilk wrote:
> > > I think an explicit flag variable is likely to be less trouble WRT
> > > maintaining compatibility in the future than a bit-field. Also I think
> > > you may as well align the struct size to something larger than a byte,
> > > either 4 or 8 bytes would make sense.
> > 
> > Ok. Will change it and make it an uint64_t secure_flag
> > variable. Later on if there are any "other" flags we can chop it down.
> 
> New patch (it also looks like the patch I sent to xen-devel to update
> the blkif.h was never merged) - so let me send right now.
> 
> BTW, it seems that the #pragma pack(push, 4) is used in the
> drivers/block/xen-blkback/common.h to compact the structures already so
> I don't think we need the aligment.

Only around the x86_32 ABI definition, working around the fact that the
original 32 bit interface was not 64 bit clean :-(. I think to ensure
that the new DISCARD structure is sensibly aligned you probably do want
it to be a multiple of 64 bits in size (32 of flags and 32 of pad would
do it).

> diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
> index 9324488..13d040e 100644
> --- a/include/xen/interface/io/blkif.h
> +++ b/include/xen/interface/io/blkif.h
> @@ -84,6 +84,10 @@ typedef uint64_t blkif_sector_t;
>   *     e07154r6-Data_Set_Management_Proposal_for_ATA-ACS2.doc
>   * http://www.seagate.com/staticfiles/support/disc/manuals/
>   *     Interface%20manuals/100293068c.pdf
> + * We also provide three extra XenBus options to the discard operation:

Who is "We" here? The frontend, backend or both? Do they both need to
agree on anything?

> + * 'discard-granularity' - Max amount of sectors that can be discarded.

... in a single request?

> + * 'discard-alignment' - 4K, 128K, etc aligment on sectors to erased.
                                          alignment

What size are the sectors which "discard-granularity" is measured in? Is
it "discard-alignment"-byte sectors or in base 512-byte sectors?

> + * 'discard-secure' - whether the discard can also securely erase data.
>   */
>  #define BLKIF_OP_DISCARD           5
>  
> @@ -107,6 +111,8 @@ struct blkif_request_rw {
>  struct blkif_request_discard {
>  	blkif_sector_t sector_number;
>  	uint64_t nr_sectors;
> +#define BLKIF_OP_DISCARD_FLAG_SECURE	(1<<1) /* ignored if discard-secure=0 */

"1<<0" is unused.

> +	uint32_t flag;
>  };
>  
>  struct blkif_request {


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